This invention relates to phase angle demodulation apparatus. In particular it relates to a phase angle demodulator with improved linearity particularly suited for demodulating the wide-dynamic range signals typically found in optical hydrophone arrays systems.
There is a demand in the oil and gas industry to improve the hit rate of locating recoverable reserves, and for increasing the percentage of oil and gas recovered from reservoirs. This has resulted in the demand for improvements in the quality of seismic surveys and in a demand for in-reservoir fluid-imaging techniques. Both these requirements demand large numbers of sensors networked together.
Streamers based on large arrays of optical hydrophones should preferably be cost-effective compared to conventional electronic arrays. Many hundreds of hydrophones are required in a single streamer and the technical specifications are demanding. In particular, the system must be capable of measuring very-low acoustic signals which arc reflected from discontinuities deep within the earth in the presence of the high-level signals resulting from multiple reflections between the sea bed and the air sea interface.
Optical hydrophones convert acoustic information to optical phase information. This is typically impressed upon an optical carrier which is frequency downshifted to an electrical carrier by mixing on a detector. Depending on the specific system architecture, the signal arrives for demodulation as either a Phase Modulated (xe2x80x9cPMxe2x80x9d) signal or a Phase Generated Carrier (xe2x80x9cPGCxe2x80x9d) signal. The process of demodulation in this context refers to the extraction of the impressed phase angle information from the signal output by an optical detector.
A demodulator for geophysical application can have a specification requirement exceeding 120 dB dynamic range where the total harmonic content must be less than 80 dB. The demodulator must therefore be capable of demodulating wide angle signals where the phase deviation covers many cycles. This requirement is difficult and places stringent demands on the linearity of the demodulation process.
An aim of the present invention is to provide a highly-linear angle-modulation demodulator. This has particular relevance for seismic streamers comprising very-large arrays of optical hydrophones.
Accordingly in one non-limiting embodiment of the present invention, there is provided an apparatus for phase angle demodulation, which apparatus comprises a demodulator and a processor in which the demodulator comprises a signal input, a reference input and a demodulator output that provides an output signal, the apparatus being such that a phase modulated signal having a carrier frequency and a phase modulation is connected to the signal input, a reference signal having a reference signal frequency is connected to the reference input, and in which a difference frequency defined as the difference in frequencies between the reference signal frequency and the carrier frequency is non-zero, and in which the processor filters the output signal.
The phase modulated signal can be any form of angle modulated signal such as a Frequency Modulated (xe2x80x9cFMxe2x80x9d) signal, a Phase Modulated (xe2x80x9cPMxe2x80x9d) signal, or a Phase Generated Carrier (xe2x80x9cPGCxe2x80x9d) signal. When demodulating a phase-generated carrier signal, it can be advantageous to combine the output from more than one demodulator each driven by a reference signal with different frequencies from each other, and where at least one of these reference frequencies is not harmonically related to the fundamental carrier frequency of the phase modulated signal.
In another an embodiment of the present invention, there is provided an apparatus for phase angle demodulation which apparatus comprises a first phase angle demodulator, a second phase angle demodulator, and a processor, in which the first phase angle demodulator comprises a first signal input, a first reference input and a first demodulator output that outputs a first output signal, the second phase angle demodulator comprises a second signal input, a second reference input and a second demodulator output that outputs a second output signal, the apparatus being such that the first demodulator output and the second demodulator output are connected to the processor, the apparatus being such that a phase modulated signal having a carrier frequency and a phase modulation is connected to the first signal input and the second signal input, a first reference signal having a first reference signal frequency is connected to the first reference input, a second reference signal having a second reference signal frequency is connected to the second reference input, and in which at least one of the first reference signal frequency and the second reference signal frequency is different from the carrier frequency, and in which the processor combines and filters the first and second output signals from the first and second phase angle demodulators in order to derive a phase demodulated signal.
The first phase angle demodulator and the second phase angle demodulator can be of the same design. It is preferred that the first reference signal and the second reference signal have a non-zero phase relationship with respect to each other. The non-zero phase relationship is preferably 90 degrees. Alternatively, the first reference signal and the second reference signal can have the same phase, and there can be a phase shift added at the input of either the first phase angle demodulator or the second phase angle demodulator.
The processor resolves phase ambiguities by combining the outputs from the first and second phase angle demodulators to generate a combined signal. The processor can be configured to resolve phase ambiguities by combining the outputs from the first and second phase angle demodulators to generate a combined signal. Preferably, the difference in the carrier frequency and the first and second reference frequencies is substantially greater than the bandwidth of the baseband signal of interest, in which case the processor can filter the combined signal to extract the desired baseband signal.
In another embodiment of the invention, there is provided an apparatus for sensing one or more parameters. The apparatus has at least one transducer which outputs a phase-modulated signal having a carrier frequency and in which the phase modulated signal is demodulated using a phase-angle demodulator according to one of the preceding embodiments. The transducer can be an electronic sensor, a piezoelectric sensor, or any other form of transducer where the resulting signal is a phase-modulated signal. The transducer can also be an optical sensor which can be an interferometer. The transducer can further be an optical fibre sensor which can be an optical fibre interferometer. The optical fibre interferometer can be an optical fibre hydrophone. There can be more than one optical fibre hydrophones configured in an array.
In another embodiment of the invention, there is provided an apparatus for marine seismic surveying comprising light sources, phase modulators, at least one array of optical fibre hydrophones, detectors and phase angle demodulators according to one of the preceding embodiments.